30-01-2013, 12:24 PM
Design of an Automatic Synchronizing Device for Dual- Electrical Generators Based on CAN Protocol
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ABSTRACT
Automatic synchronizers are widely employed to connect more than one
synchronous generator. The main task of a synchronizer is to capture the voltage,
frequency and phase differences between the generators accurately and rapidly. This
thesis introduces a new automatic synchronizer for dual generators based on CAN
“controller area network” protocol. The device consists of three independent
microcontroller modules connected with CAN protocol. One of them is called the
circuit control breaker, it is responsible for closing the circuit breaker between the
generators. The other two modules are assigned to capture the voltage, frequency and
phase differences for each generator due to a reference signals, and control the governor
of the generators to much the acceptable limits. Due to the fastness and error detection
techniques of CAN protocol, the probability of false synchronization decision is
minimized which is the main contribution of the thesis, therefore, the reliability of the
synchronization is improved. The developed automatic synchronization unit is fast, cost
effective, reliable and precise to be used for monitoring, measuring and parallel
operations of the synchronous generators.
INTRODUCTION
Electrical power system mainly consists of a generator, transmission lines, and
supplies large numbers of widely distributed loads. In many cases, there is a need to
connect more than one generator to the system. Some of the benefits of operating
multiple generators in parallel include increased reliability, expandability, flexibility,
serviceability and efficiency. Parallel operation allows operating generators around their
rated load resulting in operating with high efficiency [1, 2].
When connecting a generator to an interconnected system containing many other
generators, the voltage, phase and frequency at its terminals should meet the operating
ones. Severe damage to the generator as well as system disturbances may result if the
generator is allowed to connect to the system outside of established safe levels.
Therefore, the automatic synchronizing device plays an important role in the generator
synchronizing. For many years, the researchers have paid great attention to develop the
high performance-synchronizing device [3].
Delta Voltage Magnitude
Excessively low generator terminal voltage during the connection could cause
stability problems due to the weak magnetic circuit between the generator and the
system. On the other situation, high generator terminal voltage during the connection
will create a large reactive power flow from the system to the generator which can
damage the generator shaft and mechanical shock to the stator windings could result
from this instantaneous reactive power flow.
Therefore, it is highly important to keep the three critical synchronizing parameters
within acceptable limits. A protection should be provided to the generator during the
synchronization process. This can be done by measuring the generator parameters
before it is physically connected to the system. There are several methods available for
generator parameters measurements; most of them can be categorized into either
hardware based or software based methods [6].
Problem statement
Synchronization of two generators or more means that their characteristics should
be matched as closely as possible before the generators are connected together. They
may be rotating at different frequencies. This difference in rotation is called “slip
frequency”. It is also desired that, when the coupling circuit breaker is closed, the
relative phase-angle difference between the two generators is at or near zero [7].
Further, when the Circuit breaker is closed as in Figure 1.1, the generator’s phase-angle
displacement will be taken to zero instantaneously and the generator’s speed will be
instantaneously matched to that of the other generator. The output voltages of the dual
generator should be equal. The closer the speed is matched and the smaller the phaseangle
and voltage difference, the less mechanical stress is placed on the generator.
Stator
The stator or the armature consists of made of thin laminations of highly
permeable steel in order to reduce the core losses as in Figure 2.2.
A stator frame holds and groups the stator laminations, it may be of cast iron or
fabricated from mild steel plates. The frame is designed not to carry the flux but to
provide mechanical support to the synchronous generator [21].
The slots inside the stator are to house thick armature conductors (coils or
windings).
To form a balanced poly-phase winding, the armature conductors are
symmetrically arranged.
Synchronization
Generators are removed or connected from service due to several factors such as
variations in load, maintenance and emergency outages. Each time that a generator is
connected to a power system, it must be synchronized with it before the interconnecting
breaker can be closed.
Definition
Synchronizing, in its simplest form, is the process of electrically connecting and
matching dual generators to each other as shown in Figure 2.5. To be precise,
synchronizing is the act of matching the voltage magnitude, phase angle and frequency
of the first generator to the second generator values.
Generator damage
When the generator is connected to the power system, the electrical and
mechanical systems are tied together. Prior to closing the generator breaker during
synchronizing, the angular velocity of the rotating magnetic field and therefore the
frequency of the voltage induced in the stator are governed by the rotor speed [23].
When the breaker is closed the frequency of the power system govern the
rotating magnetic field. So the rotor and prime mover will be forced to match their
speed and position to be or become identical with the power system. If the speed and position of the rotor are closely matched at the instant the generator is connected to the
power system, the transient torque required bringing the rotor and prime mover into
synchronism is acceptable.